Airflow immediately adjacent to, and influenced, by the airfoil is called the boundary layer. Airflow at the base of the boundary layer is in contact with the airfoil and not moving relative to it, by convention the `top` of the boundary layer is the outer limit, not literally its `top` as opposed to `bottom`. The airflow gradually increases in speed until it reaches the free-stream velocity at the limit of the boundary layer. The boundary layer includes two types of air flow: steady-state (smooth) and turbulent (overlaid ripple). Steady flow is generally maintained near the leading edges of an airfoil, transforming to turbulent flow further aft. The boundary layer transition has a strong influence on airfoil lift and drag characteristics, and significant changes can be caused by light wetting of the airfoil surface (by rain or de-icing fluid, for example). The transition point is a mathematical function of Reynold's Number, which is impractical to present in real-time. At high angles of attack the boundary layers starts to separate from the airfoil, usually from the trailing edges first, leaving a region of turbulent separated flow. As this region of flow separation becomes more widespread, the airfoil suffers a significant reduction in lift coefficient and is said to "stall".
Airfoils generally produce increased lift as the angle to the relative airflow increases up to a limit called critical angle-of-attack (often called the stalling angle). Under ideal conditions, this critical angle-of-attack is constant, and has led to the use of fuselage mounted angle-of-attack (AoA) transmitters to apprise the pilot of airfoil performance. Numerous accidents have proved the AoA data is invalid when airfoil icing, leading edge contamination, miss-set lifting devices (flaps, slats, and the like), or airframe damage are present, which influence the critical angle-of-attach. Conventional stall warning systems cannot function during take off, because the wing's AoA is set geometrically by the aircraft's undercarriage during the ground roll. These systems start to function during the take off rotation, when stall margins are at their slimmest, yet airspeed is too high for a safe abort. In summary, the critical AoA information is most likely to be compromised, distorted or erroneous, under the very conditions that this information is most required.
At high speeds just above the normal cruising speed of a modern airliner, the Critical Mach Number (Mcrit) is reached, where the fastest local airflow around the airfoil first reaches sonic velocity. As Mcrit is exceeded shock waves form which significantly affect the aircraft's performance and flying qualities. These effects are a joint function of Mach Number and angle-of-attack. The common provision of Machmeter and AoA indicators to the pilot, does not provide any clear indication of the combined effect of these factors, as there is no simple way for the pilot to synthesize the relationship between the two to accurately predict the onset of compressibility effects on flight manoeuvrability.
The boundary layer transition, critical angle of attack, and critical Mach Number effectively define the performance limits of any airfoil, which have the common feature of varying degrees of turbulent flow which may be measured by instant invention.